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// Special functions -*- C++ -*-
// Copyright (C) 2006-2023 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
/** @file tr1/legendre_function.tcc
* This is an internal header file, included by other library headers.
* Do not attempt to use it directly. @headername{tr1/cmath}
*/
//
// ISO C++ 14882 TR1: 5.2 Special functions
//
// Written by Edward Smith-Rowland based on:
// (1) Handbook of Mathematical Functions,
// ed. Milton Abramowitz and Irene A. Stegun,
// Dover Publications,
// Section 8, pp. 331-341
// (2) The Gnu Scientific Library, http://www.gnu.org/software/gsl
// (3) Numerical Recipes in C, by W. H. Press, S. A. Teukolsky,
// W. T. Vetterling, B. P. Flannery, Cambridge University Press (1992),
// 2nd ed, pp. 252-254
#ifndef _GLIBCXX_TR1_LEGENDRE_FUNCTION_TCC
#define _GLIBCXX_TR1_LEGENDRE_FUNCTION_TCC 1
#include <tr1/special_function_util.h>
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
#if _GLIBCXX_USE_STD_SPEC_FUNCS
# define _GLIBCXX_MATH_NS ::std
#elif defined(_GLIBCXX_TR1_CMATH)
namespace tr1
{
# define _GLIBCXX_MATH_NS ::std::tr1
#else
# error do not include this header directly, use <cmath> or <tr1/cmath>
#endif
// [5.2] Special functions
// Implementation-space details.
namespace __detail
{
/**
* @brief Return the Legendre polynomial by recursion on degree
* @f$ l @f$.
*
* The Legendre function of @f$ l @f$ and @f$ x @f$,
* @f$ P_l(x) @f$, is defined by:
* @f[
* P_l(x) = \frac{1}{2^l l!}\frac{d^l}{dx^l}(x^2 - 1)^{l}
* @f]
*
* @param l The degree of the Legendre polynomial. @f$l >= 0@f$.
* @param x The argument of the Legendre polynomial. @f$|x| <= 1@f$.
*/
template<typename _Tp>
_Tp
__poly_legendre_p(unsigned int __l, _Tp __x)
{
if (__isnan(__x))
return std::numeric_limits<_Tp>::quiet_NaN();
else if (__x == +_Tp(1))
return +_Tp(1);
else if (__x == -_Tp(1))
return (__l % 2 == 1 ? -_Tp(1) : +_Tp(1));
else
{
_Tp __p_lm2 = _Tp(1);
if (__l == 0)
return __p_lm2;
_Tp __p_lm1 = __x;
if (__l == 1)
return __p_lm1;
_Tp __p_l = 0;
for (unsigned int __ll = 2; __ll <= __l; ++__ll)
{
// This arrangement is supposed to be better for roundoff
// protection, Arfken, 2nd Ed, Eq 12.17a.
__p_l = _Tp(2) * __x * __p_lm1 - __p_lm2
- (__x * __p_lm1 - __p_lm2) / _Tp(__ll);
__p_lm2 = __p_lm1;
__p_lm1 = __p_l;
}
return __p_l;
}
}
/**
* @brief Return the associated Legendre function by recursion
* on @f$ l @f$.
*
* The associated Legendre function is derived from the Legendre function
* @f$ P_l(x) @f$ by the Rodrigues formula:
* @f[
* P_l^m(x) = (1 - x^2)^{m/2}\frac{d^m}{dx^m}P_l(x)
* @f]
* @note @f$ P_l^m(x) = 0 @f$ if @f$ m > l @f$.
*
* @param l The degree of the associated Legendre function.
* @f$ l >= 0 @f$.
* @param m The order of the associated Legendre function.
* @param x The argument of the associated Legendre function.
* @f$ |x| <= 1 @f$.
* @param phase The phase of the associated Legendre function.
* Use -1 for the Condon-Shortley phase convention.
*/
template<typename _Tp>
_Tp
__assoc_legendre_p(unsigned int __l, unsigned int __m, _Tp __x,
_Tp __phase = _Tp(+1))
{
if (__m > __l)
return _Tp(0);
else if (__isnan(__x))
return std::numeric_limits<_Tp>::quiet_NaN();
else if (__m == 0)
return __poly_legendre_p(__l, __x);
else
{
_Tp __p_mm = _Tp(1);
if (__m > 0)
{
// Two square roots seem more accurate more of the time
// than just one.
_Tp __root = std::sqrt(_Tp(1) - __x) * std::sqrt(_Tp(1) + __x);
_Tp __fact = _Tp(1);
for (unsigned int __i = 1; __i <= __m; ++__i)
{
__p_mm *= __phase * __fact * __root;
__fact += _Tp(2);
}
}
if (__l == __m)
return __p_mm;
_Tp __p_mp1m = _Tp(2 * __m + 1) * __x * __p_mm;
if (__l == __m + 1)
return __p_mp1m;
_Tp __p_lm2m = __p_mm;
_Tp __P_lm1m = __p_mp1m;
_Tp __p_lm = _Tp(0);
for (unsigned int __j = __m + 2; __j <= __l; ++__j)
{
__p_lm = (_Tp(2 * __j - 1) * __x * __P_lm1m
- _Tp(__j + __m - 1) * __p_lm2m) / _Tp(__j - __m);
__p_lm2m = __P_lm1m;
__P_lm1m = __p_lm;
}
return __p_lm;
}
}
/**
* @brief Return the spherical associated Legendre function.
*
* The spherical associated Legendre function of @f$ l @f$, @f$ m @f$,
* and @f$ \theta @f$ is defined as @f$ Y_l^m(\theta,0) @f$ where
* @f[
* Y_l^m(\theta,\phi) = (-1)^m[\frac{(2l+1)}{4\pi}
* \frac{(l-m)!}{(l+m)!}]
* P_l^m(\cos\theta) \exp^{im\phi}
* @f]
* is the spherical harmonic function and @f$ P_l^m(x) @f$ is the
* associated Legendre function.
*
* This function differs from the associated Legendre function by
* argument (@f$x = \cos(\theta)@f$) and by a normalization factor
* but this factor is rather large for large @f$ l @f$ and @f$ m @f$
* and so this function is stable for larger differences of @f$ l @f$
* and @f$ m @f$.
* @note Unlike the case for __assoc_legendre_p the Condon-Shortley
* phase factor @f$ (-1)^m @f$ is present here.
* @note @f$ Y_l^m(\theta) = 0 @f$ if @f$ m > l @f$.
*
* @param l The degree of the spherical associated Legendre function.
* @f$ l >= 0 @f$.
* @param m The order of the spherical associated Legendre function.
* @param theta The radian angle argument of the spherical associated
* Legendre function.
*/
template <typename _Tp>
_Tp
__sph_legendre(unsigned int __l, unsigned int __m, _Tp __theta)
{
if (__isnan(__theta))
return std::numeric_limits<_Tp>::quiet_NaN();
const _Tp __x = std::cos(__theta);
if (__m > __l)
return _Tp(0);
else if (__m == 0)
{
_Tp __P = __poly_legendre_p(__l, __x);
_Tp __fact = std::sqrt(_Tp(2 * __l + 1)
/ (_Tp(4) * __numeric_constants<_Tp>::__pi()));
__P *= __fact;
return __P;
}
else if (__x == _Tp(1) || __x == -_Tp(1))
{
// m > 0 here
return _Tp(0);
}
else
{
// m > 0 and |x| < 1 here
// Starting value for recursion.
// Y_m^m(x) = sqrt( (2m+1)/(4pi m) gamma(m+1/2)/gamma(m) )
// (-1)^m (1-x^2)^(m/2) / pi^(1/4)
const _Tp __sgn = ( __m % 2 == 1 ? -_Tp(1) : _Tp(1));
const _Tp __y_mp1m_factor = __x * std::sqrt(_Tp(2 * __m + 3));
#if _GLIBCXX_USE_C99_MATH_TR1
const _Tp __lncirc = _GLIBCXX_MATH_NS::log1p(-__x * __x);
#else
const _Tp __lncirc = std::log(_Tp(1) - __x * __x);
#endif
// Gamma(m+1/2) / Gamma(m)
#if _GLIBCXX_USE_C99_MATH_TR1
const _Tp __lnpoch = _GLIBCXX_MATH_NS::lgamma(_Tp(__m + _Tp(0.5L)))
- _GLIBCXX_MATH_NS::lgamma(_Tp(__m));
#else
const _Tp __lnpoch = __log_gamma(_Tp(__m + _Tp(0.5L)))
- __log_gamma(_Tp(__m));
#endif
const _Tp __lnpre_val =
-_Tp(0.25L) * __numeric_constants<_Tp>::__lnpi()
+ _Tp(0.5L) * (__lnpoch + __m * __lncirc);
const _Tp __sr = std::sqrt((_Tp(2) + _Tp(1) / __m)
/ (_Tp(4) * __numeric_constants<_Tp>::__pi()));
_Tp __y_mm = __sgn * __sr * std::exp(__lnpre_val);
_Tp __y_mp1m = __y_mp1m_factor * __y_mm;
if (__l == __m)
return __y_mm;
else if (__l == __m + 1)
return __y_mp1m;
else
{
_Tp __y_lm = _Tp(0);
// Compute Y_l^m, l > m+1, upward recursion on l.
for (unsigned int __ll = __m + 2; __ll <= __l; ++__ll)
{
const _Tp __rat1 = _Tp(__ll - __m) / _Tp(__ll + __m);
const _Tp __rat2 = _Tp(__ll - __m - 1) / _Tp(__ll + __m - 1);
const _Tp __fact1 = std::sqrt(__rat1 * _Tp(2 * __ll + 1)
* _Tp(2 * __ll - 1));
const _Tp __fact2 = std::sqrt(__rat1 * __rat2 * _Tp(2 * __ll + 1)
/ _Tp(2 * __ll - 3));
__y_lm = (__x * __y_mp1m * __fact1
- (__ll + __m - 1) * __y_mm * __fact2) / _Tp(__ll - __m);
__y_mm = __y_mp1m;
__y_mp1m = __y_lm;
}
return __y_lm;
}
}
}
} // namespace __detail
#undef _GLIBCXX_MATH_NS
#if ! _GLIBCXX_USE_STD_SPEC_FUNCS && defined(_GLIBCXX_TR1_CMATH)
} // namespace tr1
#endif
_GLIBCXX_END_NAMESPACE_VERSION
}
#endif // _GLIBCXX_TR1_LEGENDRE_FUNCTION_TCC
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